The field of the invention is the manufacture of phthalic anhydride and the invention is particularly concerned with the manufacture of phthalic anhydride from a mixture of o-xylol and naphthalene in the gas phase.
U.S. Pat. Nos. 4,077,984 and 4,472,587; the disclosures of which are incorporated herein by reference, disclose the state of the art of manufacturing phthalic anhydride by the oxidation of o-xylol and naphthalene in the gas phase.
The oxidation of o-xylol, naphthalene, or mixtures thereof, in the gas phase to form phthalic anhydride is known and referred to many times in the literature. The oxidation of mixtures of o-xylol and naphthalene has also been described. Thus, in accordance with U.S. Pat. No. 4,472,587 the feed of o-xylol is substituted by naphthalene, as the age of the catalyst increases. This does not involve losses of production or quality. It is also known that the oxidation of mixtures of o-xylol and naphthalene produces clearly higher yields of phthalic anhydride than the conversion of the individual hydrocarbons alone as disclosed in J. Appl. Chem. USSR 41 (1968), pages 2 223 and 2 224. Due to this fact and other advantages the mixed oxidation route is a process of particular commercial interest.
Phthalic anhydride is manufactured on a large scale from o-xylol, naphthalene or mixtures thereof. In the gas phase oxidation of o-xylol the preheated hydrocarbon is atomized into hot process air having a temperature of about 170.degree. C., and this mixture of o-xylol and air is subjected to oxidation. On the other hand, the processes for the oxidation of naphthalene employ the so-called evaporator principle. In the case of the evaporator principle a primary flow of air is charged with hydrocarbon by being introduced into hot molten naphthalene at about 140.degree. C. After dilution with a further amount of secondary air this mixture of naphthalene and air is subjected to catalytic oxidation. The two processes accordingly require individually different process lay-outs in a plant, depending on the nature of the raw material as disclosed by H. Suter in Phthalsaureanhydrid und seine Verwendung, Darmstadt 1972, page 51. Accordingly, a plant designed for the oxidation of o-xylol is not suitable for the employment of naphthalene without additional expedients, and vice versa.
Processes are known for the gas phase oxidation of mixtures of o-xylol and naphthalene, wherein the evaporator principle for naphthalene on the one hand and of the injection of o-xylol on the other hand have been preserved. The mixtures of hydrocarbon and air are generated in separate parallel process lines according to the process principles peculiar to each. The lines are then combined and fed jointly to the reactor for catalytic gas phase oxidation. This procedure for mixed oxidation was developed from the process of naphthalene oxidation, supplemented by those apparatus parts which permit an additional atomization of o-xylol.
The flexibility in relation to the raw materials constitutes an important criterion when evaluating a process as disclosed by H. Suter, i.b.i.d., page 62. A process which can convert o-xylol and naphthalene under process conditions which as far as possible are identical and which is reliable in operation remains the declared objective of such process commercial development. The feeding of naphthalene as a mixture with o-xylol whilst dispensing with the evaporator principle complies with this concept also in respect to safety, because the evaporator due to its volume and the amount of products contained therein, is subject to special risks, particularly in the Low Energy Process.
The possibility of injecting atomized naphthalene as in the o-xylol process has been mentioned in the literature such as disclosed by H. Suter, i.b.i.d., page 49. However, no process based on such technology exists to date. This is due to a number of difficulties in conducting the process resulting from the particular chemical and physical properties of naphthalene. The problems arise particularly, in conducting this process whenever the hydrocarbon mixture injected into the hot process air, contains more than 20 parts by mass of naphthalene. With higher naphthalene proportions the mixed oxidation results in he formation of organic deposits in the catalyst bed which in turn after a very short time result in differential pressure rises in the reactor combined with the need to reduce the naphthalene injection or to stop the reaction.